Direct Imaging of Highly Anisotropic Photogenerated Charge Separations on Different Facets of a Single BiVO4 Photocatalyst

Spatially resolved surface photovoltage spectroscopy (SRSPS) was employed to obtain direct evidence for highly anisotropic photogenerated charge separation on different facets of a single BiVO₄ photocatalyst. Through the controlled synthesis of a single crystal with preferentially exposed {010} facets, highly anisotropic photogenerated hole transfer to the {011} facet of single BiVO₄ crystals was observed. The surface photovoltage signal intensity on the {011} facet was 70 times stronger than that on the {010} facets. The influence of the built‐in electric field in the space charge region of different facets on the anisotropic photoinduced charge transfer in a single semiconductor crystal is revealed.     

Unprecedented Eighteen‐Faceted BiOCl with a Ternary Facet Junction Boosting Cascade Charge Flow and Photo‐redox

Exposure of anisotropic crystal facets allows the directional transfer of photoexcited electrons (e^?) and holes (h⁺), for spatial charge separation. High‐index facets with a high density of low‐coordinated atoms always serve as reactive catalytic sites. However, preparation of multi‐facets or high‐index facets is highly challenging for layered bismuth‐based photocatalysts. Herein, we report the preparation of unprecedented eighteen‐faceted BiOCl with {001} top facets and {102} and {112} oblique facets via a hydrothermal process. Compared to the conventional BiOCl square plates with {001} top facets and {110} lateral facets, the eighteen‐faceted BiOCl has highly enhanced photocatalytic activity for H₂ evolution and hydroxyl radicals (^.OH) production. Theoretical calculations and photodeposition results disclose that the of eighteen‐faceted BiOCl has a well‐matched {001}/{102}/{112} ternary facet junction, which provides a cascade path for more efficient charge flow than the binary facet junction in BiOCl square plates.     

Facet Engineering on WO3 Mono-Particle-Layer Electrode for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) performance of WO₃ photoanodes for water splitting is heavily influenced by the orientation of crystal facets. In this work, mono-particle-layer electrodes, assembled by particulate WO₃ square plates with highly uniform alignment along the (002) facet, improved PEC water oxidation kinetics and stability. Photo-deposition of Au along the cracks formed on the surface of the plates, which are the edges of {110} facets, was found to further enhance electron collection efficiency. Combination of these two strategies allowed the facet-engineered WO₃ electrode to produce significantly higher efficiencies in charge separation and transfer than the electrode prepared without facet orientation. This work has provided a facile route for fabricating a structurally designed WO₃ photoelectrode, which is also applicable to other regularly shaped semiconductor photocatalysts with anisotropic charge migration.     

铁电极化对层状Bi2MoO6光催化性能的增强

通过对铁电光催化剂Bi_2MoO_6和聚甲基丙烯酸甲酯(PMMA)构建的有机-无机复合膜材料施加电场极化,来探究铁电极化对Bi_2MoO_6光催化剂活性提升的影响。未极化的Bi_2MoO_6在光照40 min时降解罗丹明B(RhB)的效率为57.6%,在光照150min时对双酚A(BPA)的降解效率为33.4%。在15 V电压下极化1.5 h的Bi_2MoO_6材料在相同条件下降解罗丹明B和双酚A的效率分别达到98.1%和79.2%,光催化活性得到了较大的提升。光催化活性提升的原因归因于内部电场的增强。未极化的Bi_2MoO_6的内电场的铁电畴是无序、分布不均匀的,光生载流子非常容易发生体内复合。当外加电场极化Bi_2MoO_6时,Bi_2MoO_6的铁电畴趋于有序,极化方向趋于同一方向,表面一侧(C+区)产生正电荷,在另一侧(C-区)产生负电荷,从C-区指向C+区的极化电场推动光生电子和空穴分别迁移到C+和C-区域。这一过程促使光生电荷载流子快速从体内迁移至表面,提高和延长了光生载流子的分离效率和寿命,导致光催化活性的提升。     

Enhancing the Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet‐Induced Spontaneous Separation of Photogenerated Electrons and Holes

Recently, it has been proven that directional flow of photogenerated charge carriers occurs on specific facets of TiO₂ nanocrystals. Herein, we demonstrate that the photocatalytic activity of anatase TiO₂ nanocrystals in both photoreduction and photooxidation processes can be enhanced by selectively depositing Pt nanoparticles on the {101} facets, which strengthens spontaneously surface‐induced separation between photogenerated electrons and holes in the photocatalysis process. An optimal ratio of the oxidative {001} facets to the reductive {101} facets exists with regard to the photocatalysis of the faceted TiO₂ nanocrystals, and this is crucial for balancing the recombination and redox reaction rates of photogenerated electrons and holes. The present work might help us gain deeper insight into the relation between the specific surface of semiconductor photocatalysts and their photocatalytic activities and provides us with a new route to design photocatalysts with high photocatalytic activity.     

Enhancing Built-in Electric Fields for Efficient Photocatalytic Hydrogen Evolution by Encapsulating C60 Fullerene into Zirconium-Based Metal-Organic Frameworks

High-efficiency photocatalysts based on metal-organic frameworks (MOFs) are often limited by poor charge separation and slow charge-transfer kinetics. Herein, a novel MOF photocatalyst is successfully constructed by encapsulating C₆₀ into a nano-sized zirconium-based MOF, NU-901. By virtue of host-guest interactions and uneven charge distribution, a substantial electrostatic potential difference is set-up in C₆₀@NU-901. The direct consequence is a robust built-in electric field, which tends to be 10.7 times higher in C₆₀@NU-901 than that found in NU-901. In the catalyst, photogenerated charge carriers are efficiently separated and transported to the surface. For example, photocatalytic hydrogen evolution reaches 22.3 mmol g⁻¹ h⁻¹ for C₆₀@NU-901, which is among the highest values for MOFs. Our concept of enhancing charge separation by harnessing host-guest interactions constitutes a promising strategy to design photocatalysts for efficient solar-to-chemical energy conversion.     

Manipulation of charge carrier flow in Bi4NbO8Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi₄NbO₈Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds.

An efficient spatial separation of photoexcited carriers is demonstrated in a layered-oxyhalide nanoplate by controlling the direction of carrier flows upon Rh cocatalyst loading, which leads to drastic photocatalytic H₂ evolution activity.     

Two-Dimensional GaTe/Bi2Se3 Heterostructure: a Promising Direct Z-scheme Water Splitting Photocatalyst

Among various photocatalytic materials, Z-scheme photocatalysts have drawn tremendous research interest due to high photocatalytic performance in solar water splitting. Here, we perform extensive hybrid density functional theory calculations to explore electronic structures, interfacial charge transfer, electrostatic potential profile, optical absorption properties, and photocatalytic properties of a proposed two-dimensional (2D) small-lattice-mismatched GaTe/Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} heterostructure. Theoretical results clearly reveal that the examined heterostructure with a small direct band gap can effectively harvest the broad spectrum of the incoming sunlight. Due to the relative strong interfacial built-in electric field in the heterostructure and the small band gap between the valence band maximum of GaTe monolayer and the conduction band minimum of Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} nanosheet with slight band edge bending, these photogenerated carriers transfer via Z-scheme pathway, which results in the photogenerated electrons and holes effectively separating into the GaTe monolayer and the Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} nanosheet for the hydrogen and oxygen evolution reactions, respectively. Our results imply that the artificial 2D GaTe/Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} is a promising Z-scheme photocatalyst for overall solar water splitting.     

Bismuth Gadolinium Oxychloride with a Remarkable Visible-Light-Responsive O2 Evolution Activity Promoted by Iodine Doping

Visible-light-responsive bismuth-based oxyhalide has recently attracted extensive attention, however, the promotion of its charge separation is still challenging. Herein, we introduce iodine into Bi₂GdO₄Cl to synthetize I-doped Bi₂GdO₄Cl (denoted as yI−Bi₂GdO₄Cl, 0≤y≤2). The incorporation of I⁻ ions is found to enhance light absorption and to accelerate charge separation by combining various characterizations such as density functional theory calculation, photoelectrochemical test, electrochemical impedance spectroscopy, photoluminescence spectrum, and open-circuit voltage decay. The O₂-evolving performances of 1I−Bi₂GdO₄Cl with optimized dopant concentration of I⁻ ion and IrO₂ loaded 1I−Bi₂GdO₄Cl are tremendously enhanced by ca. 4 and 45 times compared to pristine Bi₂GdO₄Cl. Notably, The O₂ evolution rate reaches as high as 154.8 μmol ⋅ h⁻¹ with an apparent quantum efficiency of ∼1.1 % at 420 nm. The synthetic iodine-doped photocatalyst remains stable after long-term photoreaction, demonstrating its potential in the field of photocatalysis.     

Promoting Photocatalytic H2 Evolution through Retarded Charge Trapping and Recombination by Continuously Distributed Defects in Methylammonium Lead Iodide Perovskite

Inspired by its great success in the photovoltaic field, methylammonium lead iodide perovskite (MAPbI₃) has recently been actively explored as photocatalysts in H₂ evolution reactions. However, the practical application of MAPbI₃ photocatalysts remains hampered by the intrinsically fast trapping and recombination of photogenerated charges. Herein, we propose a novel strategy of regulating the distribution of defective areas to promote charge-transfer dynamics of MAPbI₃ photocatalysts. By deliberately designing and synthesizing the MAPbI₃ photocatalysts featuring a unique continuation of defective areas, we demonstrate that such a feature enables retardation of charge trapping and recombination via lengthening the charge-transfer distance. As an outcome, such MAPbI₃ photocatalysts turn out to achieve an impressive photocatalytic H₂ evolution rate as high as 0.64 mmol ⋅ g⁻¹ ⋅ h⁻¹, one order of magnitude higher than that of the conventional MAPbI₃ photocatalysts. This work establishes a new paradigm for controlling charge-transfer dynamics in photocatalysis.     

Effects of Spatial Constraints and Brønsted Acid Site Locations on para‐Terphenyl Ionization and Charge Transfer in Zeolites

The locations of Brønsted acid sites (BAS) in the channels of medium‐pore zeolites have a significant effect on the spontaneous ionization of para‐terphenyl (PP₃) insofar as spatial constraints determine the stability of transition states and charge‐transfer complexes relevant to charge separation. The ionization rates and ionization yield values demonstrate that a strong synergy exists between the H⁺ polarization energy and spatial constraints imposed by the channel topology. Spectroscopic and modeling results show that PP₃ incorporation, charge separation, charge transfer and charge recombination differ dramatically among zeolites with respect to channel structure (H‐FER, H‐MFI, H‐MOR) and BAS density in the channel. Compartmentalization of ejected electrons away from the initial site of ionization decreases dramatically the propensity for charge recombination. The main mode of PP₃^.+ decay is hole transfer to form AlO₄H^.+ ??? PP₃ charge‐transfer complexes characterized by intense absorption in the visible range. According to the nonadiabatic electron‐transfer theory, the small reorganization energy in constrained channels explains the slow hole‐transfer rate.     

Effect of the cathode structure on the electrochemical performance of anode-supported solid oxide fuel cells

The study elementarily investigated the effect of the cathode structure on the electrochemical performance of anode-supported solid oxide fuel cells. Four single cells were fabricated with different cathode structures, and the total cathode thickness was 15, 55, 85, and 85 μm for cell-A, cell-B, cell-C, and cell-D, respectively. The cell-A, cell-B, and cell-D included only one cathode layer, which was fabricated by ( \textLa_0.74 \textBi_0.10 \textSr_0.16 )\textMnO_{3 - d} \left( {{\text{La}}_{0.74} {\text{Bi}}_{0.10} {\text{Sr}}_{0.16} } \right){\text{MnO}}_{{3 - \delta }} (LBSM) electrode material. The cathode of the cell-C was composed of a ( \textLa_0.74 \textBi_0.10 \textSr_0.16 )\textMnO_{3 - d} - ( \textBi_0.7 \textEr_0.3 \textO_1.5 ) \left( {{\text{La}}_{0.74} {\text{Bi}}_{0.10} {\text{Sr}}_{0.16} } \right){\text{MnO}}_{{3 - \delta }} - \left( {{\text{Bi}}_{0.7} {\text{Er}}_{0.3} {\text{O}}_{1.5} } \right) (LBSM–ESB) cathode functional layer and a LBSM cathode layer. Different cathode structures leaded to dissimilar polarization character for the four cells. At 750°C, the total polarization resistance (R _p) of the cell-A was 1.11, 0.41 and 0.53 Ω cm² at the current of 0, 400, and 800 mA, respectively, and that of the cell-B was 1.10, 0.39, and 0.23 Ω cm² at the current of 0, 400, and 800 mA, respectively. For cell-C and cell-D, their polarization character was similar to that of the cell-B and R _p also decreased with the increase of the current. The maximum power density was 0.81, 1.01, 0.79, and 0.43 W cm⁻² at 750°C for cell-D, cell-C, cell-B, and cell-A, respectively. The results demonstrated that cathode structures evidently influenced the electrochemical performance of anode-supported solid oxide fuel cells.     

Substituent Effects on the Photocatalytic Properties of a Symmetric Covalent Organic Framework

Symmetric covalent organic framework (COF) photocatalysts generally suffer from inefficient charge separation and short-lived photoexcited states. By performing density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, we find that partial substitution with one or two substituents (N or NH\begin{document}$_2$\end{document}) in the linkage of the representative symmetric COF (N\begin{document}$_0$\end{document}-COF) gives rise to the separation of charge carriers in the resulting COFs (\emph{i.e}., N\begin{document}$_1$\end{document}-COF, N\begin{document}$_2$\end{document}-COF, (NH\begin{document}$_2$\end{document})\begin{document}$_1$\end{document}-N\begin{document}$_0$\end{document}-COF, and (NH\begin{document}$_2$\end{document})\begin{document}$_2$\end{document}-N\begin{document}$_0$\end{document}-COF). Moreover, we also find that the energy levels of the highest occupied crystal orbital (HOCO) and the lowest unoccupied crystal orbital (LUCO) of the N\begin{document}$_0$\end{document}-COF can shift away from or toward the vacuum level, depending on the electron-withdrawing or electron-donating characters of the substituent. Therefore, we propose that partial substitution with carefully chosen electron-withdrawing or electron-donating substituents in the linkages of symmetric COFs can lead to efficient charge separation as well as appropriate HOCO and LUCO positions of the generated COFs for specific photocatalytic reactions. The proposed rule can be utilized to further boost the photocatalytic performance of many symmetric COFs.     

The Role of Polarization in Photocatalysis

Semiconductor photocatalysis as a desirable technology shows great potential in environmental remediation and renewable energy generation, but its efficiency is severely restricted by the rapid recombination of charge carriers in the bulk phase and on the surface of photocatalysts. Polarization has emerged as one of the most effective strategies for addressing the above‐mentioned issues, thus effectively promoting photocatalysis. This review summarizes the recent advances on improvements of photocatalytic activity by polarization‐promoted bulk and surface charge separation. Highlighted is the recent progress in charge separation advanced by different types of polarization, such as macroscopic polarization, piezoelectric polarization, ferroelectric polarization, and surface polarization, and the related mechanisms. Finally, the strategies and challenges for polarization enhancement to further enhance charge separation and photocatalysis are discussed.     

Semi-chemical interaction between graphitic carbon nitride and Pt for boosting photocatalytic hydrogen evolution

Owing to the exorbitant overpotential and serious carrier recombination of graphitic carbon nitride （gC₃N₄）,noble metal （NM） is usually served as the H₂evolution co-catalyst.Although the NM （such as Pt）nanoparticles can reduce the H₂evolution overpotential,the weak van der Waals interaction between Pt and g-C₃N₄makes against the charge transfer.Herein,the solvothermal method is developed to achieve semi-chemical interaction betwee...     

In situ preparation of Bi2O3/(BiO)2CO3 composite photocatalyst with enhanced visible-light photocatalytic activity

A Bi₂O₃/(BiO)₂CO₃ (BO/BOC) composite photocatalyst was in situ prepared via calcinating (BiO)₂CO₃. The as-prepared Bi₂O₃/(BiO)₂CO₃ composites displayed enhanced photocatalytic activity for the degradation of RhB under visible light. The structure–activity relationship between catalyst structure and properties was investigated by SEM, XRD, XPS, FTIR, BET, DRS and photoelectrochemical tests. Apart from the increased absorption of visible light, the accelerated charge separation and transfer was achieved via the intimate contact and matched band structure between Bi₂O₃ and (BiO)₂CO₃. The formation of heterogeneous structures could promote the production of reactive oxygen species (·O₂^?) and eventually improve the photocatalytic performance for the removal of organic contaminants. This heating treatment strategy might be extended for improving light absorbance and charge carriers separation for other UV-based photocatalysts.

     

Ferroelectrics in Photocatalysis

The rapid development of industrialization and population has brought water, air-pollution and energy crises. Solar-driven catalysis is expected to relieve these issues. However, limited by poor light harvesting, serious charge recombination of semiconductors, and high surface reaction barriers, the low efficiency of solar conversion is far from satisfactory for industrial needs. Ferroelectrics are considered to be promising photocatalysts to overcome these shortcomings. Herein, perovskite ferroelectrics such as BaTiO₃, PbTiO₃, BiFeO₃ and LiNbO₃, layered bismuth-based ferroelectrics like Bi₂WO₆ and Bi₂MoO₆, and other ferroelectrics are introduced, and their crystal structure, polarity source and synthetic method are highlighted. Subsequently, research progress in ferroelectrics for photocatalysis is summarized, including pollution degradation, water splitting and CO₂ reduction. Finally, the current challenges and future prospects of ferroelectric photocatalysts are provided. The purpose of this review is not only to provide a timely summary for the application of ferroelectrics in photocatalysis, but also to present deep insight and a guideline for future research work into ferroelectrics.     

Rapid Charge Transfer Endowed by Interfacial Ni-O Bonding in S-scheme Heterojunction for Efficient Photocatalytic H2 and Imine Production

Cooperative coupling of H₂ evolution with oxidative organic synthesis is promising in avoiding the use of sacrificial agents and producing hydrogen energy with value-added chemicals simultaneously. Nonetheless, the photocatalytic activity is obstructed by sluggish electron-hole separation and limited redox potentials. Herein, Ni-doped Zn_0.2Cd_0.8S quantum dots are chosen after screening by DFT simulation to couple with TiO₂ microspheres, forming a step-scheme heterojunction. The Ni-doped configuration tunes the highly active S site for augmented H₂ evolution, and the interfacial Ni−O bonds provide fast channels at the atomic level to lower the energy barrier for charge transfer. Also, DFT calculations reveal an enhanced built-in electric field in the heterojunction for superior charge migration and separation. Kinetic analysis by femtosecond transient absorption spectra demonstrates that expedited charge migration with electrons first transfer to Ni²⁺ and then to S sites. Therefore, the designed catalyst delivers drastically elevated H₂ yield (4.55 mmol g⁻¹ h⁻¹) and N-benzylidenebenzylamine production rate (3.35 mmol g⁻¹ h⁻¹). This work provides atomic-scale insights into the coordinated modulation of active sites and built-in electric fields in step-scheme heterojunction for ameliorative photocatalytic performance.     

Synthesis of {100} Facet Dominant Anatase TiO2 Nanobelts and the Origin of Facet‐Dependent Photoreactivity

Sword‐like anatase TiO₂ nanobelts exposed with 78 % clean {100} facets were synthesized and the facet‐dependent photoreactivity of anatase TiO₂ was investigated. By quantitative comparison with the reference {001} facets, the {100} facets possessed about ten‐times higher active sites density than that on {001} facets, resulting in higher photoreaction efficiency. After the active sites density normalization, the {100} and {001} facets exhibited distinct wavelength‐dependent photocatalytic performance, attributed to the anisotropic electronic structures in TiO₂ crystals.     

Insights into the photocatalytic mechanism of the C4N/MoS2 heterostructure: A first-principle study

Constructing heterostructures by combining COFs and TMD is a new strategy to design efficient photocatalysts for CO₂ reduction reaction (CO₂RR) due to their good stability, tunable band gaps and efficient charge separation. Based on the synthesis of completely novel C₄N−COF in our previous reported work, a new C₄N/MoS₂ heterostructure was constructed and then the related structural, electronic and optical properties were also studied using first principle calculations. The interlayer coupling effect and charge transfer between the C₄N and MoS₂ layer are systematically illuminated. The reduced band gap of the C₄N/MoS₂ heterostructure is beneficial to absorb more visible light. For the formation of type-II band alignment, a built-in electric field appears which separates the photogenerated electrons and holes into different layers efficiently and produces redox active sites. The band alignment of the heterostructure ensures its photocatalytic activities of the whole CO₂ reduction reaction. Furthermore, the charge density difference and charge carrier mobility confirm the existence of the built-in electric field at the interface of the C₄N/MoS₂ heterostructure directly. Finally, the high optical absorption indicates it is an efficient visible light harvesting photocatalyst. Therefore, this work could provide strong insights into the internal mechanism and high photocatalytic activity of the C₄N/MoS₂ heterostructure and offer guiding of designing and synthesizing COF/TMD heterostructure photocatalysts.